The invention relates to a particle-optical apparatus which includes
(a) an object carrier for an object to be irradiated by means of the apparatus, and
(b) an array of particle-optical columns, each of which is provided with:
at least one particle source for producing a limited particle beam of electrically charged particles,
a focusing device for forming a focus of the particle beam in the vicinity of the object carrier.
An apparatus of this kind is known from the published European patent application EP 0 289 278 A2.
The apparatus described in the cited patent application is arranged to write patterns on a substrate by means of electron beams in order to manufacture semiconductor circuits. Therein, the substrate to be written is arranged on an object carrier which faces the array of particle-optical columns. In the context of the present application a particle-optical column is to be understood to mean an assembly consisting of a particle source and a focusing device. The particle source produces a beam of electrically charged particles (generally an electron beam) which is focused by the focusing device that is usually referred to as the objective. The electron beam focus thus formed is situated on the substrate, so that a sharp electron spot is projected onto the substrate. The desired pattern is written by displacement of the spot. Even though no indication as regards the limiting of the electron beam can be derived from the cited European application, the beam is always limited by nature.
Generally speaking, apparatus for writing patterns by means of electron beams are constructed so as to include a plurality of particle-optical columns in order to increase the production rate in comparison with an apparatus provided with one particle-optical column only. Such columns operate in parallel during writing. An apparatus of this kind could be configured in such a manner that all columns have the same effect, that is, that they produce exactly the same current in the same spot size. However, the pattern to be written may contain details of different magnitude. In that case the small details can be written by means of an electron spot having a small diameter whereas the larger details can be written with a spot having a larger diameter. This offers the advantage that in a larger electron spot usually a larger current will be present, so that the writing of comparatively large details can take place at a speed that is higher than the speed of writing while using a small spot, thus enabling optimization of the production rate of the objects to be described. In other words, due to lens defects, for example chromatic and spherical aberration and interaction (Coulomb repulsion) of the electrons in the beam, an electron spot cannot be made arbitrarily small for a given current in the beam.
In order to achieve said advantage of optimum production rate, the known assembly is provided with columns that are capable of producing beams with mutually different currents. The known assembly of columns producing electron beams is mounted on a straight carrier arm which is arranged over the substrate to be written. The columns that are mounted on one arm differ from one another in as far as the magnitude of their beam current is concerned. In the cited European patent application this effect is achieved in that an electron source having a comparatively large emissive surface is provided so as to produce a beam with a large current; this possibility is described therein notably with reference to FIG. 6D. It is also described (notably with reference to FIG. 3) that two electron emitters that are arranged close to one another can operate simultaneously, so that these sources together produce a beam with a comparatively large current.
This known method of varying the beam current within an array of columns has the drawback that increasing of the emissive surface generally does not result in an increase of the beam current by the same factor; the beam current is increased by a substantially smaller factor under the influence of said lens defects and electron interaction.
It is an object of the invention to provide a particle-optical apparatus of the kind set forth in which the variation of the beam current can be optimized. To this end, the apparatus in accordance with the invention is characterized in that the columns are also provided with a beam limiting aperture which is situated between the associated particle source and the associated focusing device, and
the array of particle-optical columns is subdivided into at least a first group of columns and a second group of columns, the columns in the first group being provided with a beam limiting diaphragm of a first diameter and the columns in the second group being provided with a beam limiting diaphragm of a second diameter, said first and second diameters being mutually different.
The invention is based on the recognition of the fact that the beam current can be controlled better by limiting the beam by means of an aperture (in the form of a diaphragm) arranged between the source and the objective (that is, the focusing device) than by increasing the emissive surface of the electron source. In the latter case the lens defects of the objective, such as the chromatic aberration and the spherical aberration, cannot be involved in the optimization process and the spot size is determined entirely by the sum of the effects of the Coulomb interaction and said lens defects.
Each column of the first group and the second group in an embodiment of the particle-optical apparatus according to the invention is provided with a further particle lens for varying the magnification of the relevant column in co-operation with the associated focusing device.
This step offers the advantage that a further degree of freedom is obtained for influencing the beam current. The magnification of the system, that is, the ratio of the size of the electron spot on the object to be written to the actual emissive surface, can thus be adjusted as desired.
The further particle lenses in a preferred embodiment of the particle-optical apparatus according to the invention are arranged to vary, in co-operation with the associated focusing device, the magnification of the relevant column while keeping constant the distance between the focusing device and the object carrier.
This step offers considerable advantages from a manufacturing point of view, because the individual columns may now have the same physical dimensions, apart from the beam limiting diaphragm which is often mounted as a separate part any way. The array of columns can now be mounted on a unity carrier whose appearance is independent of the number and the variety of the columns that differ in respect of spot size. Moreover, only one size of column need be manufactured and stocked.
The further particle lens in a further embodiment of the particle-optical apparatus in accordance with the invention is arranged between the associated particle source and the associated beam limiting aperture. As a result of this step the further particle lens is given the function of a condenser lens so that the beam current in the electron spot can be optimized thereby by control of the magnification of the system, while the distance between the focusing device and the object carrier can still remain the same.